Solvent Tuning Excited State Structural Dynamics in a Novel Bianthryl

Symmetry breaking charge separation (SBCS) is central to photochemical energy conversion. The widely studied 9,9-bianthryl (9,9′BA) is the prototype, but the role of bianthryl structure is hardly investigated. Here we investigate excited state structural dynamics in a bianthryl of reduced symmetry, 1,9-bianthryl (1,9′BA), through ultrafast electronic and vibrational spectroscopy. Resonance selective Raman in polar solvents reveals a Franck–Condon state mode that disappears concomitant with the rise of ring breathing modes of radical species. Solvent-dependent dynamics show that CS is driven by solvent orientational motion, as in 9,9′BA. In nonpolar solvents the excited state undergoes multistep structural relaxation, including subpicosecond Franck–Condon state decay and biexponential diffusion-controlled structural evolution to a distorted slightly polar state. These data suggest two possible routes to SBCS; the established solvent driven pathway in rapidly relaxing polar solvents and, in slowly relaxing media, initial intramolecular reorganization to a polar structure which drives solvent orientational relaxation.

In the following reviewer and editor comments are normal typeface and shown in blue, our responses are black italic and new text is shown in red (as it also appears in the revised manuscript for reviewer convenience).

Reviewer 1:
Recommendation: This paper is publishable subject to minor revisions noted. Further review is not needed.
Comments: In the manuscript titled 'Solvent Tuning Excited State Structural Dynamics in a Novel Bianthryl', the authors have reported a novel anthracene dimer exhibiting SBCS. The authors have employed transient absorption and stimulated Raman spectroscopy to understand the structural dynamics in different solvents. In non-polar solvent, the bianthryl system is found to undergo structural relaxation in the excited state to a polar emissive state. The results in the manuscript are interesting and the manuscript is worth publishing in JPC Letters after considering the suggestions.
Reply: Thank you. We believe the additions suggested (below) materially improved the manuscript.

Reply: We have added to the second lines of the introduction where SBCS is introduced:
, a topic which continues to excite wide interest. 3. Spectroelectrochemical experiments or chemically oxidized or reduced data are needed to verify the ion signal.

Reply:
We have analysed both the anthracene and 1,9'BA in concentrated H 2 SO 4 and added the following in the SI Figure S20 (below) which is referred to in the text as follows:  Where Q f = fluorescence quantum yield of 1,9'BA; Q r = quantum yield of Anthracene reference; A= absorbance (at 350 nm); I = integrated emission area; n = refractive index. The recovered quantum yield was 0.54. Figure 19: Emission spectra of anthracene and 1,9'BA for the measurement of fluorescence quantum yield.

Editor:
1) TOC Graphic: Provide a TOC image per journal guidelines (2 in x 2 in; on the same page as the abstract) with the heading "TOC Graphic" above the graphic.

Reply: Done
2) Abstract: Shorten the abstract to 150 words or fewer.

Reply: Done
3) Title: In both the main manuscript file and the Supporting Information, set the title in title case, with the first letter of each principal word capitalized.
Reply: Done 4) References: In both the main file and the supporting information, fix the style of all references to use JPCL formatting (check all references carefully). ***JPC Letters reference formatting requires that journal references should contain: () around numbers, author names, article title (titles entirely in title case or entirely in lower case), abbreviated journal title (italicized), year (bolded), volume (italicized), and pages (first-last). Book references should contain author names, book title (in the same pattern), publisher, city, and year. Websites must include date of access.

Reply:
The references have been updated using the JPCL style 5) Supporting Information Statement: The supporting-information (SI) paragraph in the main file is incomplete. Please list each item (including graphics) that can be found in the SI-for-Publication file individually.

Reply:
The text now reads: Supplementary Information Supplementary information describes the details of the experimental methods, additional figures and tables specifically: Experimental details on absorption, fluorescence, transient absorption and femtosecond stimulated Raman; details of computation of excited and ground state structures by DFT and TDDFT; data analysis and global fitting; temperature dependent emission spectra; tables of measured spectral transition wavelengths as a function of solvent; table and figure of fluorescence lifetimes; figures and analysis of solvent dependent transient absorption; table of data for rotational diffusion analysis and associated graphs; details of treatment of femtosecond stimulated Raman data; details of DFT and TDDFT calculations of Raman spectra; atomic coordinated from DFT and TDDFT calculations; NMR spectra of the synthesised 1,9BA. 6) Supporting Information: Please number SI pages in the following format: "S1, S2…"